What is pyrite?
Pyrite is a golden yellow mineral with a shiny metallic lustre. The chemical composition is iron sulfide (FeS2). Pyrite is the most common sulphide mineral, it forms at both high and low temperatures and usually occurs in small quantities within igneous, metamorphic, and sedimentary rocks globally.
Pyrite is actually really common, so common in fact, that geologists generally consider it to be a ubiquitous mineral. With a lot of it around, that means it can be mined and used for many applications , more on that later.
Fools Gold
There’s a famous nickname for pyrites of ‘fools gold’. This is because of the deceptive gold colour of the mineral and metallic appearance and lustre which has fooled so many into thinking it must really be gold. But, it often could be. The two materials pyrite and gold are often found co-located together in some deposit layers, so in fact, pyrite can often be mined for its gold deposits. The most obvious way to tell the difference is that gold is definitively metallic, malleable and incredibly softer than pyrite - which tends to be more brittle.
How To Identify Pyrite
Pyrite has a unique look about it. It is angular, almost cuboid clusters in appearance, or octahedrons. It has a shiny brassy sheen, or lustre to it. It is hard and brittle and often has black streaks running through it.
The only common mineral that has properties similar to pyrite is marcasite. With an orthorhombic crystal structure and the same chemical composition, it is a dimorph of pyrite. Marcasite is a pale brass colour and does not have the same brassy yellow color of pyrite, instead sometimes it comes with a slight tint of green. Marcasite, with a slightly lower specific gravity of 4.8, is more brittle than pyrite.
Where Can Pyrite Be Found?
Although it can be fairly easy to identify, pyrites, however, is not so easy to find. Pyrite is, however, weakly magnetic. Pyrite tends to be abundant within geothermal mineral deposits and among coal beds. It is mined from carbonate rocks and vein deposits. Italy and China are the largest exporters of pyrites, but it is also found in large quantities on the borders of Wisconsin, Minnesota, and Iowa in the United States.
Uses of Pyrite
Pyrite is composed of both iron and sulfur; hence the name iron pyrites, although it is not regarded as a good source of iron, despite being primarily composed of it. Other minerals such as hematite and magnetite are used for iron production.
Pyrite as a Gold Carrying Ore
As mentioned earlier, gold is often found within or alongside pyrite as they tend to form together in the same rock types, in similar conditions. So in some deposits, small amounts of gold can occur as inclusions and substitutions within pyrite itself.
So much so, that some pyrites can contain around 0.25% or more of gold by weight. The small fraction is offset by the value of the included gold, so it is often worth mining for that reason. While it’s not a guaranteed revenue, if done efficiently enough it can be profitable.
Sulphur and Sulphuric Acid
Not as popular in the modern era, but pyrites were mined as a source of sulfur. Sulfur production now mostly comes as a byproduct of oil and gas drilling.
Gemstones
Although not a gemstone in the true sense, pyrites can be fashioned into all manner of jewellery, but mainly in the form of faceted beads. Not as popular now, but in the mid-to-late 1800s it was highly popular in the United States and Europe, owing to its pleasing lustre and visually impressive appearance. However, it does have a tendency to tarnish when in daily use.
Amber Coloured Glass
By far the most common use for pyrites in the modern age, and one that African Pegmatite specialises in is in the processing of pyrite for use in amber coloured glass. These are the yellow - almost gold to light brown coloured glassware are often found in the consumer and beverage markets.
Pyrite is mixed with other glass elements to produce this golden yellow-brown appearance. The amber chromophore is responsible for this phenomenon and is a ferric iron centre coordinated tetrahedrally by three oxygen species and a sulfide. Glassmakers utilise the amber chromophore and other pigments to manipulate the glass redox number and hence the properties of the final glass and manufacturing characteristics(1). This addition provides benefits in addition to colour:
Reduces ultraviolet light penetration
Amber glass bottles are excellent at providing UV protection for their contents. The prevention of UV radiation passing through the glass reduces the likelihood of food spoiling due to oxidative processes.
Protects against blue light
Blue light has both positive and negative influences. However, for the contents of amber glass, the primary focus is on the negative effect on food. In particular, the interaction food has with bacteria. The photochemical effect of blue light has been associated with the promotion of growth of bacteria. Using amber glass helps prevent this growth by reducing the transmission of blue light through the glass. When considering blue bottles, it’s worth noting that only amber bottles can prevent blue light.
As a safer alternative to other materials for food storage
Like many other glass related products, amber glass can reduce the risk of toxic chemicals leaking into the container contents (known as leaching) Some plastics and other materials are less safe than amber glass in this regard.
Brake Pads
Due to its tough but abrasive nature, iron pyrites are used in the manufacture of vehicle and machinery brake pads. Although brake pads wear down, pyrite enables longer life of the pads themselves. This is achieved by grinding down the pyrite into fine particles and binding them with other elements to form the required brake pad shape.
In brake pads, perhaps counterintuitively, pyrite is used as a lubricant, which is a subclass of frictional additive. The addition of pyrite to the pad causes modulation to the hardness - friction ratio, which is a crucial factor. Performance will be inhibited by a pad that is so hard it could also be brittle. At high temperatures and pressures, brittle solids will fracture. In a safety critical environment such as brake pads, the presence of pyrite prevents this from happening. Pyrite’s high thermal stability is a further reason why it is a good candidate in brake pad design(1). and can often comprise up to 30% of the weight of the brake pad. Its thermal stability aids in heat dispersion. Leading automotive parts manufacturers all use pyrite in their brake pads(2).
As a frictional additive, pyrite is one of a suite of compounds added to ensure the uniformity of braking. Other compounds used are abrasives and fillers, with their combined effects coming together to produce an effective and uniform braking system that is safe and reliable over long use periods. Brake dust often has a metallic sheen to it, partly due to the presence of pyrite.
The Manufacturing of Cast Iron
As mentioned earlier, pyrite is not regarded as a good source of iron itself and therefore is not used. It is, however, a good source of sulfur(3). Pyrite can be added to molten iron before casting as a source of sulfur in a technique called inoculation(4). The addition of sulfur to the iron typically occurs with pyrite at 100 mesh, and overall acts to aid machinability.
The level of residual sulphur is reduced when pyrites are melted in foundries, primarily due to the use of scrap steel and low sulphur content. It has been determined that melting does not respond easily to inoculants if the sulphur level is less than 0.04%. Therefore to maintain the level in a range, it is necessary to raise the sulphur content using ferro sulfur.
When making other kinds of iron and steel, one popular method of ensuring easier machining is by adding ‘engineering inclusions’. These additives are utilised as they may cause noticeable reductions in machining forces required and in wear to tooling(5). Manganese is a popular choice of additive to iron and steel. In this type of production, sulfides (from pyrite) are transformed into engineering inclusions, forming manganese sulfides. These are regarded as optimal inclusions. The greater sulfur content, the greater the size of the inclusions. In modern set ups, sulfur contents well in excess of 0.2 weight% are easily tolerated.
Hot shortness can be a problem in some grey iron casting systems, despite iron production generally having a wide tolerance of sulfur levels. Hot shortness results in cracking and breaking of the metal, rather than simple deformation which can be remedied by heat and/or machining. Engineering inclusions - particularly of the manganese sulfide variety - prevent this.
Hobby Radios
Up until recent times, pyrite was used as a mineral detector in radio receivers and is still used by crystal radio hobbyists to this day as it’s the most sensitive material detector they can depend upon for performance. Commercially, this use of pyrite fell out of use when vacuum tubes (which have since replaced themselves) became a more modern reliable method.
Photovoltaic (Solar) Panels
More recent efforts are working toward thin-film solar cells made entirely of pyrite. When used along with copper sulfide, pyrite could be a low-cost, non-toxic and abundant material as an alternative for the manufacture of solar panels. In fact, researchers are working on a completely pyrite manufactured photovoltaic panel of thin-film cells which may be seen as a revolutionary upgrade to Solar Panels.
Pyrite has an indirect bandgap of ca. 0.95 eV and a direct bandgap of 1.05-1.10 eV, which is comparable to silicon (1.10 eV). Pyrite boasts a solar absorption coefficient that is two orders of magnitude greater than silicon(6). Such electronic properties show promise in their own right, comparing favourably to silicon. Thin layers of pyrite can be used - as thin or thinner than silicon - due to its broad absorbance over the entire visible spectrum(7). Despite promising early testing in the 1990s, as a single crystal solar cell, early research showed low solar generation efficiencies. Despite this setback, researchers have since used pyrite in a perovskite solar cell, where pyrite truly comes into its own as a highly efficient hole transport medium - providing solar conversion efficiencies of up to 13.3%. In the popular dye sensitised solar cells category, pyrite has been used to replace platinum at the (counter)electrode - saving money whilst maintaining efficiency. Finally, the cost of production of each cell decreases when more pyrite is compared to silicon - which means that more cells can be produced and thus used globally.
Overall, however, pyrite suffers from not being the most efficient semiconductor material available. Computational studies point towards the shape and size of pyrite crystals being the cause of efficiency losses - or rather a lack of efficiency gains - and these can be solved with careful manipulation of the pyrite material itself(8). Carrier density values can be increased from 0.01% to 2%, with overall efficiency enhancement from 0.1% to 11% being possible by early defect detection and remediation. Such care in preparation and assembly is noted by other researchers who, whilst agreeing that p-type pyrite is an excellent material for solar cell manufacture, insist that specific temperature spray pyrolysis can ensure an optimised band gap - and therefore better performance(9).
A more detailed overview of pyrite in solar cells is available elsewhere on this website
Tribocatalysis
Tribocatalysis is a new and emerging field where mechanical energy is converted to chemical energy, to effect the removal of organic pollutants from solution. Researchers have studied whether pyrite might perform well in this setting - owing to its unique chemical and electronic structures, and that it is already a well known photocatalyst. Whilst research is limited in this area, it has been shown as effective at catalysing the decomposition of methylene blue (a common laboratory dye) under simple stirring conditions, with no conventional “wet” chemistry required(18).
Summary
- Pyrite is a naturally occurring ore of iron - though isn’t considered a good source of iron itself
- Often referred to as ‘fool’s gold’, pyrite has a pleasing appearance and is often used in decorative settings. Gold itself is sometimes co-located with pyrite in the ground.
- Industrial applications include in brake pads and grinding wheels, in addition to as a source of sulfur to enhance malleability in iron production
- It is used as a colourant/pigment in glass manufacture, being largely responsible for the amber chromophore, and its inclusion in glass can aid in preserving the contents of the glass by minimising light exposure
As a leading miner, processor and supplier of iron pyrite, African Pegmatite is the go-to partner for high quality pyrites, processed and milled to a customer’s exacting specifications. African Pegmatite is an industry leader, with the experience, reach and processing ability to ensure quality every time.
References
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5 I. Riposan and T. Skaland, Modification and inoculation of cast iron, in: D. M. Stefanescu (ed.) Cast Iron Science and Technology, ASM International, Novelty, Ohio, United States, 2017
6 H. Opitz, Proc. Int. Prod. Eng. Res. Conf., 1963, 107
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8 A. Kumar et al., Optical Quantum Elect., 2023, 55, 244
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